This invention relates to a method for suppressing crystallization of a fluoride glass article by treating the surface(s) of the glass article with a liquid agent.
Fluoride glasses are excellent in light transmittance over a wide range of wavelength extending from the ultraviolet region to the infrared region and hence have been attracting interest as optical materials for use in optical windows, optical fibers, laser oscillators, etc.
Most of hitherto developed fluride glasses employ ZrF.sub.4 or ALF.sub.3 as the principal component and contain at least one other metal fluoride such as, for example, BaF.sub.2, CaF.sub.2, MgF.sub.2, PbF.sub.2 and/or NaF. Usually a fluoride glass article is produced by melting a mixture of metal fluorides and casting the melt in a mold. A solid fluoride glass body is obtained by rapid cooling of the melt in the mold. Then the surfaces of the fluoride glass body are smoothed and cleaned usually by a water polishing method.
In general fluoride glasses are relatively small in the difference between the glass transition temperature (T.sub.g) and the crystallization temperature (T.sub.x), T.sub.x -T.sub.g, and considerably low in thermal stability compared with oxide glasses. For example, when a fluoride glass is softened by heating for the purpose of fiberization, the heated fluoride glass is liable to crystallize from the surface by the influence of moisture and/or contaminants adhering to the glass surface. If crystallization occurs the mechanical strength of the fluoride glass fiber greatly reduces, so that the fiberization process can hardly provide a fiber product of stable quality. Besides, in the case of an optical fiber of a fluoride glass crystallization causes an increase in the scattering loss and consequently an increase in the transmission loss.
There are proposals of surface treatments for suppressing crystallization of fluoride glass articles. A typical example is a sort of etching treatment using a ZrOCl.sub.2 --HCl--H.sub.2 O system as etching liquid. However, the application of an aqueous liquid agent to fluoride glasses raises problems. Compared with oxide glasses, fluoride glasses are very liable to dissolve in water. When a fluoride glass surface makes contact with water the glass surface becomes susceptible to cracking by corrosion. If a fluoride glass is heated while moisture is adhering to the glass surface it is likely that the moisture reacts with the fluoride glass to form some oxides which become a cause of crystallization.
Also there are proposals of treating fluoride glass surfaces with a gas, but this method is low in efficiency because of using a gas-solid reaction and has not exhibited a practically satisfactory effect.